The area looks external to the CRT - possible conductive white RTV sealant. Sony and others used it for isolation of the focus pin, but the RTV gets dirty with dust over time and the combination of HV and dust leads to problems. Sony had the tech take off the RTV coating between the plastic CRT base and the CRT, and use their part-numbered RTV sealant. We used RTV3145 or GC's Silicone Sealant for Electronics with 100% success.

It's unlikely, but it's possible that another problem such as an internal short in the CRT, or a bad flyback could be causing excessive voltage on that CRT pin. Try running it with the CRT socket unplugged to see if it still arcs on either the socket, or the end of the CRT.

It's unlikely, but it's possible that another problem such as an internal short in the CRT, or a bad flyback could be causing excessive voltage on that CRT pin. Try running it with the CRT socket unplugged to see if it still arcs on either the socket, or the end of the CRT.

Exactly..........
Could also be the spark gap in the CRT socket
All pins will have spark gaps almost always built into the socket.
Sometimes they are separate little rectangles with a slice
in them. Usually fail from too much focus voltage ( FBT ).

I removed the circuit board from the neck of the TV - no arcing on the socket. No improvement with the board re-seated (in case there was any build-up of oxidation). There is no silicone or anything there that could have become conductive.

Upon observing the CRT again, the arcing appears to be just inside the glass. But I assume that isn't possible without air in the tube, so I'm guessing that the lead has broken just outside the glass, and the CRT is a goner.